First lesson in zoology : adapted for use in schools . like. One of them has alarge projection on the edge; this is called the tooth, inthe other shell there is a cavity corresponding to the cavity and the hollow on the inner side of the largetooth is, if the shell is a fresh one, filled with a dark-brown THE CLAM AND OTHER BTVALVEB SHELL-FISH. 55 Bubstance called the ligament (Fig. 58, Kg). When theclam is alive this ligament is slightly elastic, so as to allowthe valves, aided by the two great muscles within, to openand shut. The tooth in the one valve and the cavity inthe opposit
First lesson in zoology : adapted for use in schools . like. One of them has alarge projection on the edge; this is called the tooth, inthe other shell there is a cavity corresponding to the cavity and the hollow on the inner side of the largetooth is, if the shell is a fresh one, filled with a dark-brown THE CLAM AND OTHER BTVALVEB SHELL-FISH. 55 Bubstance called the ligament (Fig. 58, Kg). When theclam is alive this ligament is slightly elastic, so as to allowthe valves, aided by the two great muscles within, to openand shut. The tooth in the one valve and the cavity inthe opposite valve, together with the ligament, form thehinge of the shell. The edges of the valves are alsothickened to support the hinge, and this side of the shell iscalled the upper or dorsal side; hence the opposite edge ofthe shell is called the lower or ventral side. Thus the clam-shell has an upper and lower side, and a front and hinderend, and each side is for the most part like its shall also see that the animal within has a dorsal and Z?^. FiQ. 58.—Inside view of the clam-shell (natural size). PI, pallieil line, made bytile mantle; ndm, impression of the anterior; adm, of the posterior adductormuscle; Ug, ligament; bk, beak, ventral side, a head-end and hinder-end, and that bothsides of the animal are alike. Moreover, we notice over the hinge, outside of the shell,that each valve is full and swollen over the site of thehinge. Each swelling is somewhat like a birds beak; hencethey are called collectively the beaks. We also observe that the surface of the valves is rough andmarked with lines nearly parallel to the outer edge of thevalves. These are the lines of growth. They are not atequal distances apart, for the clam probably does not growsteadily at all times of the year, but by fits and starts. Ifwe pick up some very young clams, however, we shall see 56 FIB8T LESSONS IN ZOOLO&T. that in outline they correspond to the concentric lines ofgrowth near the beaks. We
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1894
